Electron multiplier



March 14, 1939.

T. H. CLARK ELECTRON MULT I PLIER Filed Jan., 26, 1938 INVENTOR. TREVOR H. CLARK ATTORNEY.

Patented Mar. 14, 1939 UNITED STATES PATENT OFFICE;

ELECTRON MULTI'PLIER.

Delaware Application January 26, 1938, Serial'No. 186,949

4 Claims.

M'y invention relates to electron multipliers of the type in which electronsoscillate within a cylindrical surface and repeatedly bombard the surface at a velocity sufficient to produce secondary electrons at a ratio greater than unity so that an electron multiplication takes place, and more particularly to such multipliers for use as oscillation generators of the long wave type, in which the transit time of the secondary electrons is small compared to the period of the oscillations.

In one form of such a secondary electron emission oscillator a tubular cold cathode, with the inner surface so treated that it is photosensitive and also has a high coefficient of secondary electron emission, surrounds-and is concentric with a tubular perforated anode. An alternating voltage of high frequency superimposed on the anode voltage will cause electrons emitted by the tubular photo emissive cathode to be accelerated by and to pass through the grid-like anode and impinge on the cathode surface on the other side of the anode with sufficient velocity to produce several secondary electrons for each electron, so that both primary and secondary electrons oscillate back and, forth transversely of the tubular cathode and through the grid-like anode. It has been found that such a tube lacks operational frequency stability, is difiioult to start into oscillation, is thermally unstable and also has a comparatively short life. A filamentary cathode is often included inside the anode extending through the anode along the longitudinal axis of the anode and cathode, so that it iSl directly in the path of the secondary electrons. This cathode is used to supply primary electrons to augment the primary photoelectrons used for starting. If the long filamentary cathode is at cathode potential, the transit time of the electrons across the anode space is lengthened and the efliciency of the tube decreases at higher frequencies, and if the filamentary cathode is at anode potential, some of the electrons do not reach the secondary cathode because they are caught by the filament and some reach it at such a part of the cycle that they and the secondary electrons which they produce are detrimental rather than beneficial to the operation of the device.

The object of my invention is to provide an electron multiplier of this general type which is easily started, has good stability and efiiciency, and has a longer life than multipliers of this kind constructed in the usual way. In accordance with 4 my invention the tube has 'a tubular secondary electron emitter or cathode surrounding a tubular grid-like anode concentric with and as long as the cathode, and also a small thermionic cathode, either filamentary or indirectly heated, at one end of the tubeand out of the path of travel of the secondary electrons which oscillate back and forth in the tubular cathode transversely of the longitudinal axis of the tube. The thermionic cathode mounted out of the path of the electrons produced by secondary emission eliminates many of the disadvantages which arise when a filamentary cathode extends along the axis of the tube in the path of the secondary electrons, as the secondary electrons do not strike the filament and aifect its temperature, with resulting instability of the tube. Although the thermionic electrons from the thermionic cathode at the end of the tube and out of the path of the secondary electrons reach only a fraction. of the total cathode surface, they arrive at such time as to augment the electrons otherwise liberated from the cathode, resulting in easier starting and more stable operation. The secondary electron emitter is usually a caesiated silver surface, which is adversely affected by high temperatures. of the tube and the uniformity of operation is greatly increased in accordance with my invention by providing some cooling means, such as a'water jacket, for controlling the temperature of the secondary electron emitting cathode and maintaining it cool enough, preferably below 150 (2., to preserve its secondary electron emissivity for a long period of time.

Other objects and advantages of my invention will appear from the following description, read with reference to the accompanying drawing in which Figure 1 illustrates one embodiment of my invention, and Figure 2 is a longitudinal section of another embodiment utilizing a water jacket to control the temperature of the secondary electron emitter.

The tube shown in Figure 1 comprises a highly evacuated envelope or bulb- I enclosing a hollow cylindrical or tubular secondary electron emitting col-d cathode 2 which may be of silver, silver on copper, or silver on'copper on nickel, with an inner surface of silver which is oxidized and caesiated in accordance with the usual practice so that it will have a high secondary electron emissivity and a'ratio of secondary electrons to primary electrons considerably greater than unity for low velocity bombarding electrons. Inside this unipotential tubular secondary electron emitter and concentric with it is a hollow cylindrical or'tubular grid-like unipotential anode The life 3, which may conveniently be made in the form of a helical grid with two side rods 4 and a grid wire 5 wound into a helix with its turns secured to the side rods. The secondary emitter cathode 2 and the anode may conveniently be held in pro-per relation by mica spacers 6. Secondary electrons emitted at any portion of the inner surface of the emitter 2 will flow through the anode to another portion of the surface more or less diametrically opposite the portion where they originate. V

In accordance with my invention a thermionic cathode l, which in this particular embodiment of the invention is a short piece of tungsten wire, is mounted near one end of the cathode and anode so as to be outside the path of the secondary electrons, and is preferably near the axis of the tube. The thermionic cathode is maintained, at least during starting operation, at such a temperature that it emits sufficient thermionic electrons to supply the initial electrons, which are then multiplied by the bombardment of the inner caesiated silver surface of the secondary electron emitter 2. Since the thermionic cathode is outside the path of the secondary electrons which oscillate back and forth through the anode 3, there is no instability or interference with the operation of the tube due to bombardment of the thermionic cathode by the secondary electrons and no change in space potential within the anode space by a charged conductor. The thermionic cathode may be operated at the potential of the secondary emitter, or of the anode, or at any potential between them, or at a few volts negative with reference to the secondary emitter cathode. I have found it best that the thermionic cathode be somewhat below the potential of the secondary emitter cathode for starting, and at the same potential as the secondary electron emitter cathode for stable and efficient operation. After oscillations have started the thermionic cathode may be disconnected, if desired, as the oscillations will continue without thermionic electrons from the thermionic cathode.

To produce oscillations, an alternating voltage of a suitable frequency, usually several megacycles, is impressed between the anode and secondary emitter 2. Such a voltage may be obtained from an excited oscillatory circuit 9 connected in series with a source of current, such as a battery l0. By tuning the oscillatory circuit 9 and applying to the current a certain excitation such as from another oscillator or from a spark discharge alternating voltages of the desired frequency may be impressed between the cathode 2 and the anode 3. For convenience in fixing the voltage of the thermionic cathode T with reference to the other electrodes, a voltage divider H may be used as indicated in Figure 1. If modulation is desired, an input circuit 12 may be used.

I have obtained good results with a tube constructed as shown in Figure 1 in which the secondary electron emitter 2 is a thin-walled tube about 2 /4 inches in diameter and 4 inches long made of copper with a thin silver inner surface, and the anode 3 a tubular cylindrical grid as long as the emitter 2, about 1 inches in diameter, and consisting of two 40 mil side rods :3 and a 10 mil grid wire 5 wound into a helix of three turns per inch. In this tube the thermionic cathode I is 2 mil tungsten wire about 1 cm. long. The emitter 2 was caesiated with caesium vapor generated from a pellet consisting of caesium chromate, zirconium and a little aluminum. An excess of caesium is prevented by tin oxide or similar material painted on the outer surface of the emitter 2 as is customary in photo-tubes. The thermionic cathode 1 decreased from about 350 volts to slightly less than 200 volts the anode voltage required for starting and when kept hot also stabilized the operation of the tube.

The tube is exhausted and treated in much the same way as conventional photoelectric tubes, preferably by exhausting the tube, baking at 450 C. for 30 minutes, cooling and admitting oxygen to 1 mm. pressure, oxidizing the silver surface by an ionic discharge in the low pressure oxygen, or other means known to the art, pumping out the oxygen, introducing caesium vapor so as to be spread uniformly over the oxidized silver surface,

baking for ten minutes at 160 C. to distribute caesium, baking at 250 C. until a color ranging from black to straw is obtained on the silver surface, and sealing off the tube. The best results were obtained with tubes which were operated under oscillating conditions for long periods during the pumping, which may have given a harder tube due to degassing of the anode by bombardment during operation of the tube on the pumps. With such'tubes 40 watts output has been obtained at an efliciency of 40%in the vicinity of 6 megacycles for a short time and 10 to 15 watts output can be obtained in favorable frequency ranges with a reasonably long life.

Figure 2 shows a modified form of tube in which the secondary electron emitting surface is maintained at a low temperature during operation.

' In this particular embodiment of the invention caesiated so that it has a high secondary electron emission ratio. inder l3 and concentric with it is a cylindrical grid-like anode [5 supported from the glass ends M. The anode may be water cooled if desired,

and if not, may be provided with rather heavy metal end pieces to assist in cooling it by radiation. The thermionic cathode! is mounted near one end of the tube and out of the path of the secondary electrons as in the form of tube shown in Figure 1.

The temperature of the secondary electron emitting surface is controlled by a cooling means such as a water jacket 16 through which cooling water may be circulared through inlet and outlet pipes ll. During operation the circulation of water through the water jacketmaintains the temperature of the secondary electron emitting inner surface of the electrode 1 3 at such a temperature, preferably below 150 0., that the surface will retain its secondary electron emitting properties for a long time, even though the tube is operated under rather severe conditions and with heavy loads I claim: 7

1. An electron multiplier comprising a unipotential secondary electron emitter having spaced surface portions facing each other, a gridlike anode coextensive with and positioned between said surface portions, and a thermionic Within the tubular copper cylcathode at one end of said emitter out of the path of secondary electrons flowing between said portions and through said anode.

2. An electron multiplier comprising a hollow cylindrical cold cathode with an inner surface of high secondary electron emissivity, a hollow cylindrical grid-like anode inside and concentric with said cold cathode, and a thermionic cathode out of the path of secondary electron flow from said inner surface and exposed to the interior of said cold cathode.

3. An electron multiplier comprising a hollow cylindrical cold cathode with an inner surface of high secondary electron emissivity, a hollow cylindrical grid-like anode inside and concentric with said cold cathode, means for controlling the temperature of said cold cathode, and a thermionic cathode out .of the path of secondary electron flow from said inner surface and exposed to the interior of said cold cathode.

4. An electron multiplier comprising a hollow cylindrical cold cathode with an inner surface of high secondary electron emissivity, a hollow cylindrical grid-like anode inside and concentric with said cold cathode, a water jacket surrounding said cold cathode, and a thermionic cathode out of the path of secondary electron flow from said inner surface and exposed to the interior of said cold cathode.

TREVOR H. CLARK. 

